C1q is one of the complement proteins, and is known to act in the complement activation pathways. For instance, the activation of the classical pathway is known to be triggered by binding of C1q to the Fc fragment of an immunoglobulin molecule.
It has been reported that rheumatoid arthritis (RA) patients with large quantities of blood concentration of C1q will suffer from joint destruction in the future (Non-patent Document 1). Since the activation of C1q described above is thought to be involved, development of an inhibitor of the binding between C1q and the immunoglobulin molecule is needed.
The possibility that an arginine residue on the C1q B subunit (B chain) was involved in the binding between C1q and immunoglobulin molecules has been reported (Non-patent Document 2). However, as this report was based on a prediction by a computer simulation, the actual immunoglobulin binding site was not clear. In addition, there was no detailed amino acid sequence of the binding site.
Proteins such as Protein A or Protein G, which can specifically bind to immunoglobulins, are used in the purification of immunoglobulins. However, since these proteins bind strongly to immunoglobulins, once they are bound, a harsh condition such as using a strongly acidic buffer is required for separation. For this reason, the conformation of the immunoglobulin is prone to be unfolded, and a high-affinity antibody cannot be purified.    Non-patent Document 1: Ochi T et al., Arthritis Rheum. 1988 January; 31(1): 37-73    Non-patent Document 2: Gaboriaud C et al., J Biol Chem. 2003 Nov. 21; 278(47): 46974-82. Epub 2003 Sep. 5